Why did part of my mesh suddenly turn purple in sculp mode? (Face sets)
In Sculpt mode, Face Sets allows you to create separate areas, a little bit like the Mask except you can create as many face sets as you want and you won't have to mask and unmask to switch from a face set to another, when you'll sculpt on a face set it will only affect this face set until you release the left click and click on another face set.
To paint your face sets you need to press left click. Release and left click again to create a new face set, Ctrl and left click on a face set to extend it, Shift left click to smooth the boundaries. In the Face Sets menu &gt; Init Face Sets you can choose another way to create face sets (for example By UV Seams):

Once you've painted, choose your sculpt brush and enable the Face Sets options:

It looks like once you've painted your face sets you can't enable Dyntopo without deleting the face sets, Dyntopo creates new geometry so it makes sense.
Now sculpt: